In the art, there are known methods and systems for controlling and moderating frequency. A conventional transceiver includes therein a local oscillator which, for example in radio frequency (RF) systems, is usually a frequency generating crystal. When the transceiver receives a signal in a given frequency f.sub.S, it has to shift the frequency f.sub.S of the signal to a desired frequency f.sub.d by producing an internal frequency f.sub.LO, using the local oscillator. Conventional frequency crystals are known to include a frequency shift which needs to be corrected, according to the received signal frequency f.sub.S.
Reference is now made to FIGS. 1 and 2. FIG. 1 is a schematic illustration of a prior art system, generally referenced 1, for controlling the internal frequency of a transceiver. FIG. 2 is a graphic illustration of the frequency versus voltage curve, referenced of a typical voltage-controlled-oscillator (VCO).
System 1 includes a voltage-controlled-oscillator 2 (VCO), an input frequency shift unit 5 and an output frequency shift unit 7, both connected to VCO 2, and a frequency estimator 4, connected to VCO 2 and to input frequency shift unit 5, for determining the required frequency correction.
Conventionally, a demodulator 6 is connected after the input frequency shift unit 5 and a modulator 8 is connected before the output frequency shift unit 7.
A received signal provided to the input frequency unit 5, incorporates a frequency f.sub.S. The VCO 2 generates an internal frequency f.sub.IN and provides it to the input frequency shift unit 5. The input frequency shift unit 5 produces a new signal having an intermediate frequency of f.sub.I wherein f.sub.I =f.sub.S -f.sub.IN, and provides the new signal to the frequency estimating unit 4 as well as to the demodulator 6. It will be appreciated that conventional demodulators define a maximal allowed frequency difference .DELTA.f.sub.ALLOWED between f.sub.I and f.sub.d, so that .vertline.f.sub.I -f.sub.d .vertline..ltoreq..DELTA.f.sub.ALLOWED. A conventional oscillator oscillates at a frequency which changes due to poor manufacturing quality, temperature changes and the like so that a situation where .vertline.f.sub.I -f.sub.d .vertline.I&gt;.DELTA.f.sub.ALLOWED often occurs.
The frequency estimator 4 detects the signal provided by the frequency shift unit 5. If .vertline.f.sub.I -f.sub.d &gt;.DELTA.f.sub.ALLOWED then f.sub.I has to be adjusted to equal f.sub.d. The frequency estimator 4 determines the frequency correction .DELTA.f=f.sub.d -f.sub.I, determines a respective voltage V(.DELTA.f) and provides the voltage V(.DELTA.f) to the VCO 2. Accordingly, the VCO 2 generates a corrected internal frequency f.sub.IN.
It will be noted that, as long as .vertline.f.sub.I -f.sub.d .vertline.&gt;.DELTA.f.sub.ALLOWED, the demodulator cannot properly process the incoming signal.
The modulator 8 produces a signal for transmitting at a frequency of f.sub.T and provides the signal to the output frequency shift unit 7. The VCO 2 provides an internal transmitting frequency f.sub.OUT to the output frequency shift unit 7. In turn, the output frequency shift unit 7 produces a transmittable signal bearing a frequency of f.sub.OUT +f.sub.T which is the transmitted signal b broadcasting frequency.
It is noted that f.sub.IN and f.sub.OUT are generated around a main frequency f.sub.LO, generated by the local oscillator of the VCO unit 2.
Referring now to FIG. 2, when provided with a voltage at a value of v.sub.1, the VCO 2 is expected to provide the respective frequency f. A conventional VCO, having an internal inaccuracy, may provide, for a given voltage value of v.sub.1, a frequency in the range between f.sub.MIN and f.sub.MAX. When maximum(f.sub.MAX -f,f-f.sub.MIN)&gt;.DELTA.f.sub.ALLOWED, the frequency correction cannot be deemed final.
Conventionally, after several cycles of detection and correction, the intermediate frequency f.sub.I should differ from the desired frequency f.sub.d by no more than .DELTA.f.sub.ALLOWED. Until then, the demodulator 6 cannot perform properly.
It will be appreciated that the modulator 8 is also dependent on the accuracy of the frequency provided thereto by the VCO 2.